Thermoplastic resins, particularly polyolefins, are inexpensive and have excellent rigidity, resistance to moisture and thermal resistance. Thus, they are used in a wide range of applications such as materials for automobile, materials for household electric appliances and the like.
On the other hand, while a trend to avoid soft polyvinyl chloride is growing stronger owing to the problems of hormone-disrupting substance, dioxin and the like, preference is given to polyolefins, which have flexibility and transparency. Under such circumstances, thermoplastic polyolefin-based elastomers called TPOs have excellent flexibility but no transparency, while a system prepared by adding styrene-based elastomers to PP has flexibility and transparency but is poor in rubber elasticity and expensive, thus these systems being limited in applications (Patent Documents 1 to 11).
In addition, Patent Document 12 describes a propylene•ethylene•1-butene copolymer, but does not describe a material having excellent transparency and flexibility as well as rubber elasticity.
Moreover, polypropylene has been conventionally used in a wide range of applications as a thermoplastic molding material having excellent rigidity, thermal resistance, transparency and the like. This polypropylene, however, has poor flexibility and impact resistance, and thus, soft rubber components are usually blended with polypropylene.
As such, when soft rubber components are blended with polypropylene, polypropylene compositions having improved flexibility and impact resistance are obtained, but these compositions still have a problem of thermal resistance being deteriorated. These polypropylene compositions are also desired to have improvement in low temperature heat sealability.
Hence, there has been a demand for development of a polypropylene composition having excellent flexibility and impact resistance as well as excellent thermal resistance and low temperature heat sealability.
In addition, crystalline polypropylene has excellent mechanical properties such as tensile strength, rigidity, surface hardness, impact resistance and the like, optical properties such as gloss, transparency and the like, and food hygiene properties such as non-toxicity, odorlessness and the like, and is widely used particularly in the field of food packaging. However, the film of this crystalline polypropylene shrinks when heated to the heat sealing temperature, and it is difficult to heat seal with a single layer of this film. Thus, the crystalline polypropylene film is typically provided with a heat seal layer, and this heat seal layer is generally formed from a polymer such as a low density polyethylene, a propylene•ethylene random copolymer or the like.
Meanwhile, the polymer forming such heat seal layer is required to have specific performances such as that: (1) heat sealing is possible at a much lower temperature than the base material (crystalline polypropylene film); (2) the heat seal strength is excellent, while the variation in the heat seal strength with time is small; (3) the adhesion with the base material is excellent; (4) transparency is excellent to a degree equivalent to or higher than that of the base material; (5) no blocking occurs during the storage; (6) the polymer does not adhere to a bag-making apparatus or a filling packaging machine; (7) scratch resistance is excellent; and the like.
However, conventionally known heat sealing materials do not satisfy all of these performances, and for example, the above-described low density polyethylene can be heat sealed at low temperatures, but it is poor in the heat seal strength, adhesion to the base material and transparency, and further presents problems such as liability to adhere to packaging machines, and the like.
Furthermore, propylene ethylene random copolymers satisfy the above-described performances (2) to (7), but do not satisfy the performance (1), and polypropylene composite film having a heat seal layer formed from a propylene•ethylene random copolymer has a narrow temperature width for heat sealing. Thus, if this composite film is to be heat sealed with use of an automatic packaging machine, an automatic bag-making machine or the like, the heat sealing temperature must be strictly controlled. Moreover, it has been proposed to use a blend of a propylene•ethylene random copolymer and an ethylene•α-olefin copolymer as a heat sealing material, and this blend has improved low heat sealability compared to propylene•ethylene random copolymers, but this blend is deteriorated in transparency.
Meanwhile, the Applicant of the present invention previously found that a propylene•1-butene random copolymer having a propylene content of 55 to 85% by weight and a heat of crystalline fusion of 20 to 80 J/g as measured by differential scanning calorimetry, has excellent transparency and good low temperature heat sealability, so that the copolymer is useful as a heat sealing material. And, the Applicant proposed the use of a composition comprising this propylene•1-butene random copolymer and an isotactic polypropylene, which containing the propylene•1-butene random copolymer in an amount of 50% by weight or more, as a heat seal layer for the polypropylene film (Patent Document 13). However, the heat seal layer formed from this composition has excellent low temperature heat sealability and anti-blocking properties, but is somehow poor in the anti-blocking properties and scratch resistance as compared with the above-mentioned propylene•ethylene random copolymer.
The same Applicant has also proposed a composite film having a heat seal layer for isotactic polypropylene, which is formed from a composition comprising a propylene•1-butene copolymer and a crystalline propylene•α-olefin random copolymer, and containing the propylene•1-butene copolymer in an amount of 10 to 40% by weight, as a composite film having excellent heat sealability (Patent Document 14).
However, such polypropylene film is further desired to have a property appropriate to be applied in high speed packaging, and is desired to have slip properties and anti-blocking properties, together with improved low temperature heat sealability.
JP-A-08-238733 discloses a composite film comprising a propylene•1-butene copolymer prepared with use of a metallocene catalyst, and a crystalline propylene•α-olefin random copolymer as a heat seal layer. However, there is a problem if the melting point of the propylene•1-butene copolymer is adjusted to near 70° C., the rate of crystallization is lowered, and the productivity is decreased. Furthermore, if the content of the propylene•1-butene copolymer is excessive, there occur problems such that it is likely to have poor moldability or deterioration in film appearance (Patent Document 15).
A crosslinked olefin-based thermoplastic elastomer is used as an energy and resource saving type elastomer, and is widely used in automobile parts, industrial machinery parts, electronic•electrical instrument parts, construction material and the like, particularly as a substitution of natural rubber.
The crosslinked olefin-based thermoplastic elastomer is widely known as is described in detail in the literature [A. Y. Coran et al., Rubber Chemistry and Technology, Vol. 53 (1980), p. 141] (Non-Patent Document 1).
Meanwhile, a non-crosslinked or partially crosslinked olefin-based thermoplastic elastomer is described in, for example, the above-mentioned Patent Documents 1 to 9.
Although the non-crosslinked or partially crosslinked thermoplastic elastomer has excellent rubbery properties (permanent elongation, permanent compression set, etc.), heat resistance and the like, but the elastomer has poor abrasion resistance and scratch resistance, so that the substitution for soft polyvinyl chloride is not achieved. Thus, there has been a demand for an olefin-based thermoplastic elastomer composition having excellent abrasion resistance and scratch resistance, which can substitute soft polyvinyl chloride without involving environmental problems, waste treatment problems and the like.
[Patent Document 1] JP-B-53-21021[Patent Document 2] JP-B-55-18448[Patent Document 3] JP-B-56-15741[Patent Document 4] JP-B-56-15742[Patent Document 5] JP-B-58-46138[Patent Document 6] JP-B-58-56575[Patent Document 7] JP-B-59-30376[Patent Document 8] JP-B-62-938[Patent Document 9] JP-B-62-59139[Patent Document 10] JP-A-7-149999[Patent Document 11] JP-A-8-27353[Patent Document 12] JP-A-3-200813[Patent Document 13] JP-A-54-114887[Patent Document 14] JP-B-61-42626[Patent Document 15] JP-A-08-238733[Non-Patent Document 1] Rubber Chemistry andTechnology, Vol. 53 (1980), p. 141